Absorption (electromagnetic radiation)

About: Absorption (electromagnetic radiation) is a research topic. Over the lifetime, 76674 publications have been published within this topic receiving 1381221 citations.

Visible and near infrared reflectance characteristics of dry plant materials

Abstract: Laboratory reflectance spectra of dry plant materials are distinctly different from green plant materials. The dry plant materials have diagnostic ligno-cellulose absorption features at 2·09 and in the 2·3μm region. The visible portion of spectrum is dominated by an absorption wing produced by intense blue and UV absorption. This absorption wing breaks down as plant decay progresses.

Effects of mixing on extinction by carbonaceous particles

Abstract: Reported values for the absorption cross section of particulate carbon per unit mass range from under 4 to over 20 m2/g, and the intermediate value of 10 m2/g is used by many as a standard gram-specific absorption cross section for atmospheric soot. In order to better understand the possible variations in absorption by atmospheric carbon, we reevaluated its optical properties in terms of the material composition and morphology of soot and the electrodynamics of spherules agglomerated into loose (ramiform) aggregates. Primary particles ranging in composition from paracrystalline graphite to low-density air/graphite volume mixtures are considered. The effects on extinction efficiency of aggregation and of internal mixing of carbon with sulfate are considered in detail. We also compare our results with estimates of specific absorption of internally mixed soot that are based on several homogeneous mixing rules (effective medium approximations), On the basis of our modeling of the optical properties of aggregates of graphitic carbon grains, we conclude that 10 m2/g may be over 50% too high in many cases, and we suggest that the mass absorption coefficient for the light-absorbing carbon in diesel soot at a wavelength of 0.550 μm may often be less than 7 m2/g, although variations in optical constants and, especially, the specific gravity of the absorbing material make it difficult to assign a specific numerical value. Adhesion of carbon grains to sulfate droplet surfaces is expected to enhance their absorption by no more than about 30%. Soot randomly positioned within droplets, however, can display averaged absorption enhancement factors of about 2.5–4 for hosts with refractive indices ranging from 1.33–1.53, respectively, and radii ≳0.20 μm. Nonetheless, calculations indicate that for realistic dry particle populations, αa < 10 m2/g for graphitic carbon in the atmosphere unless (1) most of it is encapsulated, and (2) the geometric mean radius of the hosts is larger than about 0.06 μm (which corresponds to a mass median diameter of 0.34 μm). These results suggest the importance of the determination of the physical state of the soot particles and their immediate environment when ascribing characteristic values for their absorption and scattering efficiencies.

Limitations in the enhancement of visible light absorption due to mixing state

Abstract: [1] Absorption by light-absorbing carbon (LAC) particles increases when the carbon is mixed with other material, and this change affects climate forcing. We investigate this increase theoretically over a realistic range of particle sizes. Perfect mixing at the molecular level often overestimates absorption. Assuming that LAC is coated by a concentric shell of weakly absorbing material, we calculate absorption by a range of realistic particle sizes and identify regimes in which absorption behaves similarly. We provide fits to amplification in five regions: (1) small cores and (2) intermediate cores, both with large shells; (3) small to intermediate cores with intermediate shells; (4) cores with growing shells; and (5) intermediate to large cores with large shells. Amplification in region 1 is highest but is physically implausible. Amplification in region 5 is constant at about 1.9 and represents an asymptote for particles with broad size distributions. Because absorption by aggregates is amplified by about 1.3 above spherical particles, and that factor is lost when particles are coated, we suggest that absorption by aged aerosol is about 1.5 times greater than that of fresh aerosol. The rate at which particles acquire sufficient coating to increase their original diameter by 60% is important in determining total absorption during their atmospheric lifetimes. Fitted amplification factors are not very sensitive to assumed refractive index of LAC and can be used even in simple models.